This invention relates to a fixing apparatus for fixing a toner image on a support medium such as a sheet of paper by heating the toner image and pressing it thereon.
Heretofore, it is well known to produce a toner image on a suitable support medium through an electrophotography method, electrostatic printing method or magnetic printing method and then to fix the formed toner image by a fixing apparatus for heating and pressing it.
There is disclosed such a fixing apparatus in U.S. Pat. No. 4,603,245, which comprises a heat-fixing roller, a halogen heater for heating the roller, a thermistor for detecting the surface temperature of the roller, and a temperature control means for controlling the surface temperature thereof by turning on or off the halogen heater. The temperature control means controls the halogen heater in such a manner, as shown in FIG. 5, that two different temperatures t.sub.1 and t.sub.2 are preset, at one of which the heater is turned off and at the other of which the heater is turned on, that when these temperatures are alternately detected, a timer is started, and that the heater is kept ON and OFF until predetermined periods of times a and b preset by the timer are up, respectively. Therefore, the temperature of the fixing roller is controlled so as to be fluctuated within a certain range of two predetermined temperatures t.sub.1 and t.sub.2. However, generally, the thermistor detects the temperature of the fixing roller with a delay time to cause the difference between a temperature detected by the thermistor and an actual surface temperature of the fixing roller. That is, even if the heater is turned on to raise the temperature of the fixing roller to a temperature higher than the predetermined temperature t.sub.1 (for example, 180.degree. C.), the temperature detected by the thermistor is lower than the predetermined temperature and the heater is kept ON. When a temperature detected by the thermistor is raised to a temperature higher than or equal to the predetermined temperature t.sub.1, the heater is turned off. However, at this time, the actual temperature of the fixing roller is raised to a temperature (for example, 200.degree. C.) higher than the predetermined temperature t.sub.1. Especially, the longer a lighting time of the heater becomes, the larger an increasing ratio, per a unit time, of the temperature of the heater becomes thereby to increase the difference between the detected temperature of the fixing roller and the actual temperature thereof. Likewise, when the heater is turned off, the actual temperature is apt to go down lower than the predetermined temperature t.sub.2. As a result, an overshoot of the temperature of the fixing roller occurs as indicated by a dotted line 0 in FIG. 5, and the temperature of the fixing roller cannot be actually controlled within a predetermined range of the two different temperatures. Therefore, a stable fixing of toner on a sheet of paper cannot be performed.